fix: Initialize async IO thread lazily (#8122)

* fix: Initialize async IO thread lazily

The goal of that change is to avoid effects of threads when they are not
actually needed.

---------

Signed-off-by: Etienne Millon <me@emillon.org>

CHANGES.md

@@ -4,6 +4,8 @@ Unreleased
 - Disable background operations and threaded console on MacOS and other Unixes
   where we rely on fork. (#8100, #8121, fixes #8083, @rgrinberg, @emillon)
 
+- Initialize async IO thread lazily. (#8122, @emillon)
+
 - Add `dune build --dump-gc-stats FILE` argument to dump Garbage Collection
   stats to a named file. (#8072, @Alizter)
 

src/dune_async_io/async_io.ml

@@ -35,6 +35,7 @@ type t =
   ; mutex : Mutex.t
   ; scheduler : (module Scheduler)
   ; mutable running : bool
+  ; mutable started : bool
   ; (* this flag is to save a write to the pipe we used to interrupt select *)
     mutable interrupting : bool
   ; pipe_buf : Bytes.t
@@ -208,10 +209,42 @@ let rec select_loop t =
       Scheduler.fill_jobs fills);
     select_loop t
 
-let t_var = Fiber.Var.create ()
+let start t =
+  let module Scheduler = (val t.scheduler : Scheduler) in
+  Scheduler.spawn_thread (fun () ->
+      Mutex.lock t.mutex;
+      Exn.protect
+        ~f:(fun () -> select_loop t)
+        ~finally:(fun () -> Mutex.unlock t.mutex))
+
+module T_var : sig
+  (** Wrap the global t_var so that it is started whenever requested. *)
+
+  val get_exn : unit -> t Fiber.t
+
+  val setup : t -> (unit -> 'a Fiber.t) -> 'a Fiber.t
+end = struct
+  let t_var = Fiber.Var.create ()
+
+  let get_exn () =
+    let+ t = Fiber.Var.get_exn t_var in
+    if not t.started then (
+      start t;
+      t.started <- true);
+    t
+
+  let setup t f =
+    Fiber.Var.set t_var t (fun () ->
+        Fiber.finalize f ~finally:(fun () ->
+            if t.started then (
+              Mutex.lock t.mutex;
+              t.running <- false;
+              interrupt t;
+              Mutex.unlock t.mutex);
+            Fiber.return ()))
+end
 
 let with_io scheduler f =
-  let module Scheduler = (val scheduler : Scheduler) in
   let t =
     let pipe_read, pipe_write =
       if not Sys.win32 then Unix.pipe ~cloexec:true ()
@@ -233,25 +266,13 @@ let with_io scheduler f =
     ; pipe_buf = Bytes.create 512
     ; interrupting = false
     ; to_close = []
+    ; started = false
     }
   in
-  let () =
-    Scheduler.spawn_thread (fun () ->
-        Mutex.lock t.mutex;
-        Exn.protect
-          ~f:(fun () -> select_loop t)
-          ~finally:(fun () -> Mutex.unlock t.mutex))
-  in
-  Fiber.Var.set t_var t (fun () ->
-      Fiber.finalize f ~finally:(fun () ->
-          Mutex.lock t.mutex;
-          t.running <- false;
-          interrupt t;
-          Mutex.unlock t.mutex;
-          Fiber.return ()))
+  T_var.setup t f
 
 let with_ f =
-  let+ t = Fiber.Var.get_exn t_var in
+  let+ t = T_var.get_exn () in
   Mutex.lock t.mutex;
   Exn.protect ~f:(fun () -> f t) ~finally:(fun () -> Mutex.unlock t.mutex)
 
@@ -267,7 +288,7 @@ let cancel_fd scheduler table fd =
            Fiber.Ivar.fill t.ivar (Error `Cancelled))
 
 let close fd =
-  let* t = Fiber.Var.get_exn t_var in
+  let* t = T_var.get_exn () in
   Mutex.lock t.mutex;
   (* everything below is guaranteed not to raise so the mutex will be unlocked
      in the end. There's no need to use [protect] to make sure we don't deadlock *)